The invention relates to a method for growing large-volume monocrystals of uniform orientation from a melt, to a device for carrying out this method and to the use of crystals prepared in this manner.
Monocrystals are characterized by the fact that they have a uniform orientation throughout their entire volume which is a prerequisite for high optical homogeneity within the entire crystal volume. For this reason, they are eminently suited for use in the optical industry or as starting material for optical components in deep-ultraviolet [DUV] photolithography, for example for steppers or excimer lasers.
The growing of monocrystals from a melt is in itself known. Text books about crystal growing, for example “Kristallzüjchtung” [The Growing of Crystals] by K. Th. Wilke and J. Bohm, which has 1088 pages, describe a wide variety of different methods for crystal growing of which the most common techniques will be mentioned briefly in the following. In principle, crystals can be grown from the gas phase, from the melt, from solutions or even from a solid phase by recrystallization or diffusion through a solid body. Such methods, however, are meant primarily for laboratory-scale work and not for large-scale industrial production. The most important large-scale melt growing processes for making crystals will be explained briefly in the following.
The Czochralski method involves dipping, with the aid of a finger-type tool, a slightly cooled seed crystal into a crucible containing molten crystal raw material and then pulling this seed crystal out slowly, preferably with rotation. In this manner, during the pulling, the seed crystal grows into a larger crystal.
The drawback of this method is that cooling produces relatively large temperature changes in the crystal resulting in stress-induced anisotropy.
By the vertical Bridgeman method, a crystal raw material is melted in a mobile melting crucible by means of a heating jacket. The crucible is then slowly lowered from the heating jacket through an axial temperature gradient produced by heaters. Or, alternatively, the crucible is stationary, and a mobile heating system is moved upward. The melt is thus cooled allowing the slow growth of an added seed crystal. In a variant of this method known as the Bridgeman-Stockbager method, a crystal is formed by slowly lowering the mobile crucible in an axial gradient between two heating jackets disposed above each other and between which exists a major temperature difference.
In the vertical gradient freeze method (VGF method), several concentric heating coils are disposed over each other around the stationary melting crucible so as to form a jacket. Each of these coils can be controlled separately. By slowly decreasing the heat output of each individual heating coil disposed around the crucible wall, the temperature is slowly reduced to below the crystallization point thus generating a radial temperature gradient along which crystal growth takes place.
In the gradient solidification method (GSM), a ring-shaped heating coil surrounding a stationary melting crucible is slowly moved downward and then upward.
Nevertheless, oriented monocrystals usually do not exhibit homogeneous optical and mechanical properties. It is desirable that such crystals be produced with a crystal orientation appropriate for a particular application. This, however, creates major problems in the production of large monocrystals, because during their growth such crystals spontaneously change their orientation, namely the position of their crystal axis. This leads to optically nonuniform crystals which do not exhibit the same light refraction in all regions.
Until now, while it was possible to produce crystals exhibiting some of these properties, it was not possible to grow large-volume uniformly oriented crystals that are free of convergences, are optically highly homogenous, exhibit high transmission and, in addition, do not discolor when exposed to a strong radiation source.
Attempts have been made by hitherto known methods, for example in the production of large calcium fluoride monocrystals, to grow the crystal in the direction of the {111} axis. This gave very low yields, however, namely only about 6-8% of the growing attempts gave a satisfactory crystal size. Because such crystal growing methods involve a process with a running time of approximately 6 weeks, and the number of such growing units is limited because of cost reasons, only low yields were achieved. Moreover, it was not possible by use of previously employed methods to produce large-volume monocrystals, particularly monocrystals extending far in all three directions in space, namely preferably round crystals with a diameter of >200 mm and a height of >100 mm, because such dimensions regularly lead to block formation within the crystal volume, namely a reorientation of the crystal axes takes place. Moreover, it has thus far not been possible to obtain satisfactorily such large crystals also in optically highly homogeneous form, namely so that their light refraction is the same in all regions. Another problem with such crystals is their radiation resistance, namely their ability not to undergo discoloration when exposed to a strong radiation source, for example a laser. This problem causes a decrease in yield, for example in the large-scale production of wafers.
It has already been attempted to produce large monocrystals by growing them in the shape of plates. EP-A-0 338 411, for example, describes an apparatus and a method for the controlled growing of large monocrystals in plate shape from a melt and by use of a melting crucible which has a rectangular cross-section and is configured so as to present two relatively wide and two relatively narrow side walls with heating devices disposed immediately adjacent to the wide sides. In this case, after the melting, the crucible is slowly lowered from the heating jacket by means of a lifting device as a result of which the crucible contents cool and crystallize. Although by this method it is possible to produce large oriented monocrystal plates, said plates do not adequately extend in all three directions in space.